106 PHYSIOLOGICAL PHYSICS. [Chap. x. 



by brass columns on a plate of mahogany, levelled by 

 three screws. The coils are arranged on each side of 

 the copper chamber, and by means of a sledge arrange- 

 ment can be caused to meet right over the chamber, 

 so that the chamber is contained in the centre of the 

 two, or the coils can be removed from the chamber. 

 In Fig. 56 the coils are represented close to one 

 another, and therefore hide the copper chamber which 

 is within them. In the upper corner of the figure 

 the magnetised ring is shown attached by the alumi- 

 nium rod to the plane mirror. In very sensitive 

 instruments the number of turns on the coils is as 

 great as 30,000. 



The features of this instrument are the arrange- 

 ments for DAMPING THE OSCILLATIONS of the needle. 

 The copper chamber is called the DAMPER. The move- 

 ment of the magnetised needle sets up induction 

 currents in the copper mass in the opposite direction 

 to the movement of the needle, and this diminishes 

 the oscillations of the needle, and causes it, after deflec- 

 tion, to come quickly to rest. The close fitting of the 

 ring to the chamber aids this action, as well as the 

 proximity of the coils to the needle. Another point 

 is that by the ring shape the inactive portion of the 

 magnet, its centre, is taken away, and the needle is 

 made stronger in proportion to its size. Now, this 

 needle is not astatic, but is made so by means of a bar 

 magnet of considerable strength, to be immediately 

 described. 



The position of the botissole should be care- 

 fully chosen. It may be placed on a strong oaken 

 shelf, fastened to a solid dry wall in front of a window, 

 brass fixings being used, and none of iron. No iron 

 structure whatever should be in the neighbourhood, 

 either about or outside of the window. If the instru- 

 ment is to be used in a laboratory on a ground floor, 

 then a pillar of concrete, with a cap of oak, and built 



